This application pertains to the art of tomographic image reconstruction and more particularly, to computerized tomographic scanners for medical diagnostic and other uses.
The invention is particularly applicable to medical diagnostic computerized axial tomographic scanners and will be described in particular reference thereto. It will be appreciated however, that the invention has broader application such as industrial flaw detectors.
Generally, a tomographic scanner comprises a scan circle to which an object to be examined may be placed. Movably mounted about the scan circle is a source of radiation so mounted and positioned that it can irradiate a planar slice of a scan circle from a number of directions within a plane. Positioned opposite the scan circle from the source of radiation is one or more radiation detectors which receive radiation which has traversed the scan circle. From the variations in the radiation intensity detected along various known paths to the scan circle, a processing unit reconstructs an image of the planar slice of the object in the scan circle. The intensity of the radiation received by each detector is a function of the intensity of the source, the mass absorption coefficient of each material between the source and the detector, and the path length of each material between the source and the detector.
The mass attenuation coefficient of almost all materials is a function of the energy of the radiation. Thus, for polychromatic radiation, the amount of attenuation is different for each of the many energy components which make up the polychromatic spectrum. This energy dependent variation in the attenuation coefficient causes problems in prior art scanners because it produces nonlinearities which distort the reconstructed image.
Another problem with prior tomographic scanners has been the dosage level with which they irradiate the patient or other object in the scan circle. Generally, the intensity of the radiation produced by the source is constant across the scan circle. Radiation of the same intensity is aimed to traverse all parts of the scan circle including the relatively long paths near the center and the relatively short paths near the periphery. Because the thickness of the object and the scan circle is generally thinner near the periphery, the radiation traverses a shorter path length than it would if if were passing through the center of the scan circle. When the path length is shorter, a lower intensity of radiation is required for reconstructing the image. Accordingly, when the entire scan circle is subject to radiation of the same intensity, part of the patient is subject to more radiation than is necessary.
Another problem with the prior art tomographic scanners has been their inability to compensate for various different nonlinearities introduced by different filter materials. The change of the polychromatic spectrum caused by different filter materials has been found to change the tomographic scanner's ability to differentiate among various materials which comprise the object or patient being examined.
In the past, others have tried to alleviate these problems by using generally flat filters. Others have used water equivalent filters which are shaped so that radiation from the radiation source passing through the water equivalent filter and the scan circle traverse an equivalent amount of water regardless of which part of the scan circle they traverse. Water has been selected because when human patients are examined, the mass attenuation coefficient of the human body is generally the same as the mass attenuation coefficient of water.
The present invention contemplates a tomographic scanner which overcomes all of the above problems and others. The present invention provides a tomographic scanner which is as simple to operate as prior scanners, but can compensate for non-linearities produced by filters of nearly any material and configuration without complex correction calculations or tune-up procedures.
In accordance with the present invention, there is provided an apparatus for reconstructing an image of a region of an object positioned within a scan circle. The apparatus comprises a source of radiation having a polychromatic spectrum for irradiating the scan circle with radiation from a plurality of directions. It also includes at least one radiation detector mounted and positioned to receive radiation from the source which has traversed the scan circle along a plurality of paths. A radiation filter is positioned between the source of radiation and the detector. A data collection means collects from the at least one detector data indicative of the intensity of radiation received by each of the radiation detectors along the plurality of paths and organizes the collected data into data lines. A beam hardness correction means adjusts the data lines for nonlinearities introduced by the polychromatic spectrum of radiation from the source of radiation. A filter correction means adjusts the data lines for nonlinearities introduced by alterations to the polychromatic spectrum by the radiation filter. The beam hardness correction means and the filter correction means are operatively connected with the data collection means. A reconstruction means which reconstructs the data lines into a representation of an image of the region is connected to the beam hardness correction means and the filter correction means for receiving data lines which have been adjusted thereby.
In accordance with a more limited aspect of the invention, the filter correction means comprises a first altering means for altering the data lines by adding a value thereto before the data line is adjusted by the beam hardness correction means and a second adjusting means for adding a value to the data line after correction by the beam hardness correction means.
In accordance with another more limited aspect of the invention, the radiation filter is constructed of a block of beryllium, aluminum, sulfur, calcium, titanium, erbium, or copper or compounds or alloys thereof with a parabolic recess.
A principal advantage of the invention is that filters of nearly any shape and material may be used in a tomographic scanner.
Another advantage of the present invention is that it reduces dosage received by the patient without degrading the reconstructed image.
A further advantage of the present invention is that improved reconstructed images are achieveable with the substitution of different radiation filter materials.